We are trying to describe in various cells, but primarily using the Ehrlich cell, the functional steps of mediated membrane transport, namely recognition and binding; secondary and subsequent responses to binding, including energy coupling; and release. Our main approach is to use modifications of amino acid structure, also analog substitution for the alkali-ion cosubstrate, in order to complete the delineation of transport systems and their modes of substrate recognition, and to describe their functional and regulatory relations, and also to detect essential intermediate events in translocation (e.g., proton transfers) not evident from simple kinetic study. Flux ratios of modified substrate to cosubstrate and to H ion will be measured, and conditions altering these will be studied. We will seek to discover from the nature and effects of structural changes the nature of the affected event in the sequence of steps in transport. From the information gained we hope to be led to understand how the components of transport systems function in normal and abnormal metabolism, and to be guided toward isolative opportunities.